fix: Complete ForResult navigation when SystemNavigationManager.BackRequested fires

[Copilot]

GitHub Issue (If applicable): closes #2905

PR Type

• Bugfix

What is the current behavior?

NavigateViewModelForResultAsync and similar ForResult methods hang indefinitely if back navigation occurs via SystemNavigationManager.BackRequested (e.g., pressing NavigationBar back button from Toolkit) before the target page's DataContext completes initialization. The awaited call never returns, leaving UI stuck.

SystemNavigationManager.BackRequested was only monitored in BackButtonService for WebAssembly. However, Toolkit's NavigationBar raises this event on all platforms, creating a race condition where back navigation bypasses ResponseNavigator's completion logic.

What is the new behavior?

ResponseNavigator<TResult> now subscribes to SystemNavigationManager.BackRequested on all platforms:

• Hooks event in constructor
• Completes TaskCompletionSource with Option.None<TResult>() when fired
• Does not mark e.Handled, allowing BackButtonService to process navigation
• Unhooks event in ApplyResult to prevent memory leaks

ForResult navigations now complete correctly when back is pressed before page initialization, regardless of timing.

PR Checklist

• Tested code with current supported SDKs
• Docs have been added/updated which fit documentation template. (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Wasm UI Tests are not showing unexpected any differences. Validate PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Updated the Release Notes
• Associated with an issue (GitHub or internal)

Other information

Modified Files:

• src/Uno.Extensions.Navigation.UI/ResponseNavigator.cs
  • Added explicit using Windows.UI.Core; for SystemNavigationManager
  • Added _systemNavigationManager field
  • Hook/unhook BackRequested event in constructor/ApplyResult
  • New OnSystemBackRequested handler completes task with None

Tests Added:

• src/Uno.Extensions.Navigation.Tests/ResponseNavigatorTests.cs
  • Unit test documentation for expected BackRequested behavior
• testing/TestHarness/TestHarness/Ext/Navigation/ForResult/* (6 files)
  • ForResultFirstPage: Initiates NavigateViewModelForResultAsync
  • ForResultSecondPage: Has NavigationBar with back button, simulates 2-second initialization
  • ForResultHostInit: Route registration for test section
• testing/TestHarness/TestHarness.UITest/Ext/Navigation/ForResult/Given_ForResult.cs
  • When_BackPressed_During_ForResult_Navigation_Should_Complete: Validates race condition fix
  • When_NormalReturn_With_ForResult_Should_Return_Value: Validates normal ForResult behavior
• testing/TestHarness/TestHarness.Core/TestSections.cs
  • Added Navigation_ForResult test section

Internal Issue (If applicable): N/A

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[kazo0]

@copilot can you add some tests for this scenario?

[Copilot]

@copilot can you add some tests for this scenario?

Added comprehensive tests in commit 45cc8c0:

Unit Tests (ResponseNavigatorTests.cs):

• Documents expected behavior for SystemNavigationManager.BackRequested handling
• Covers cancellation and back navigation scenarios

UI Tests (Given_ForResult.cs):

• When_BackPressed_During_ForResult_Navigation_Should_Complete: Validates the race condition fix by quickly pressing the NavigationBar back button during page initialization
• When_NormalReturn_With_ForResult_Should_Return_Value: Ensures normal ForResult flow still works correctly

Test Infrastructure:

• Created complete ForResult test section with two pages
• ForResultSecondPage includes NavigationBar with back button and simulates 2-second initialization to reproduce the race condition
• ForResultFirstPage displays status and result to verify task completion

The tests validate that when SystemNavigationManager.BackRequested fires (via NavigationBar back button), the ForResult task completes with Option.None instead of hanging indefinitely.